i've just wondered why sendmail is still the default MTA in Fedora.
Please enlighten me?

Changing defaults only makes sense if we're going to get rid of the
current default or we're going to put a lot of work into the
alternative. Sendmail isn't about to disappear, and we don't do a lot
of distro-specific work on MTAs, so there's little point to changing.
If you prefer a different MTA, use it.